1. Field of the Invention
The present invention relates to an error-erasure correction of data reproduced from an optical disc device, and more particularly, to a method and system for correcting errors and erasures in modulated channel data by indicating error locations at the time of demodulating the modulated channel data.
2. Description of the Related Art
An optical disc, such as a compact disc (CD) or digital versatile disc (DVD), is used for storing a large quantity of audio, video and/or other data information. When such information is recorded on the optical disc and read therefrom, noises can be generated. To correct errors due to the noises, in a DVD system a Reed-Solomon (R-S) product code is used as an error correction code, and an inner code (PI) of (182,172,11) and an outer code (PO) of (208,192,17) are included in the R-S product code. Here, “182” of the inner code or “208” of the outer code represents a length of a code word, that is, the number of symbols which form a code word. “172” of the inner code or “192” of the outer code represents a length of message or information of a code word, that is, the number of symbols that form the information of a code word. “11” and “17” each represent a minimum distance, called a minimum Hamming distance, of a code word.
A CD system uses a cross-interleave Reed-Solomon code (CIRC) as an error correction code. CIRC includes a C1 code of (32,28,5) and a C2 code of (28,24,5). In the C1 code or the C2 code, the first factor (“32” of the C1 code or “28” of the C2 code) represents the length of a code word, the second factor (“28” of the C1 code or “24” of the C2 code) represents the length of information of a code word, and the last factor, “5”, represents the minimum distance of a code word.
There is a limit to the correction of the inner code (PI) and the outer code (PO) of a R-S product code and the C1 and C2 codes of a CIRC code. The correction limit is determined by the minimum distance of a code word. For example, if the number of errors in a code word is defined as “e”, the number of erasures in the same code word as “v”, and the minimum distance of the code word as “d”, an error correction using the R-S product code for a DVD system or the CIRC code for a CD system can correct errors of the code word only if Equation 1 is satisfied.2e+v<Minimum Distance  (1)
Here, ‘error’ means that neither an error value nor an error location is known, and ‘erasure’ means that an error value is not known but an error location is known. The error value is determined by the difference between an original symbol value and an erroneous symbol value corresponding to the original symbol. The ‘error location’ is the location of an erroneous symbol. The ‘erroneous symbol’ means that an original symbol is damaged by noises produced by data processing such as recording and reproducing.
Table 1 is a summary of characteristics of a R-S product code and a CIRC code used in CD and DVD systems, respectively.
TABLE 1CorrectableCorrectableMinimumErrorErasureCodeFormatDistanceNumberNumberCD C1 Code(32, 28, 5)524CD C2 Code(28, 24, 5)524DVD PI Code(182, 172, 11)11510DVD PO Code(208, 192, 17)17816
Both the C1 code and C2 code have the minimum distance of “5”, so that with respect to the C1 and C2 codes, it is possible to correct up to two (2) errors or four (4) erasures per each code word. If there are errors and erasures together in a code word, it is possible to correct up to one (1) error and two (2) erasures.
The PI code has the minimum distance of “11”, so that it is possible to correct up to five (5) errors or ten (10) erasures in a PI code word. The PO code has the minimum distance of “17”, so that it is possible to correct up to eight (8) errors or sixteen (16) erasures in a PO code word.
The CD or DVD system uses a slicer to change an analog signal read from a CD or DVD into digital data. A conventional slicer changes an input sample signal into a binary number such as “1” (or logic high state) or “0” (or logic low state) using high and low threshold values between the two logic states. In other words, a conventional slicer uses a “soft decision method” in which if an input sample signal is smaller than the low threshold value, it outputs “0”, if the input sample signal is bigger than the high threshold value, it outputs “1”, and if the input sample signal is between the high and low threshold values, it outputs an “erasure.” Since the slicer outputs 14 bits (in the case of the CD) or 16 bits (in the case of the DVD) data to form one symbol, use of the soft decision method in the slicer causes an increase in the number of the erasures. As a result, the actual efficiency of the error correction is lowered.
For these reasons, a C1 decoder or a PI decoder in a error correction system does not use the erasure correction in the error correction of a C1 word or a PI word. An erasure flag can be obtained from the result of the error correction of the C1 or PI word in the C1 or PI decoder. A C2 decoder or a PO decoder uses the erasure flag in the erasure correction of a C2 word or a PO word. This is because the entire error correction efficiency is higher when the C2 or PO word is used for the erasure correction than when the C2 or PO word is used for the error correction.
In a conventional error correction system, therefore, it is possible to error-correct up to 2 erroneous symbols per each code word for a C1 code, and it is possible to erasure-correct up to 4 erroneous symbols per each code word for a C2 code. Similarly, it is possible to error-correct up to 5 erroneous symbols per each code word for a PI code, and it is possible to erasure-correct up to 16 erroneous symbols per each code word for a PO code.
However, for high speed optical devices such as a high speed CD-ROM and a high speed DVD-ROM, high speed data processing is required when restoring data from such media. The incidence of errors in a high speed data processing is higher than the incidence of errors in a low speed data processing.
Therefore, a need exists for an error correction system which is more efficient and effective in the error correction than a conventional error correction system in a high speed as well as a low speed data processing.